Various metal composite films (hereinafter referred to as "composite films") are developed for improving the properties of substrates or imparting functions to the substrates. Among others, composite films comprising a metal and an inorganic filler or fluorine resin filler are developed for improving the corrosion resistance, abrasion resistance, self-lubricity, adhesion, decorativeness, etc. of the surfaces of metal substrates.
Conventionally, a composite film is formed on a substrate by an electroplating process using a plating solution to which a filler has been dispersed for incorporating the filler into the metal film when the metal is deposited on the substrate.
However, since the filler is used as dispersed in the plating solution, fillers chemically reactive with the plating solution or those easily precipitated because of their excessive specific gravity are not usable in the above electroplating process. As a result, the type of filler that can be used is very restricted.
Further, since the proportion of the filler in the plating solution is difficult to change greatly, the formulation of the composite film to be formed can not be controlled as desired. Moreover, the above process is not suitable for commercial-scale operations from the viewpoint of reproducibility. In particular, plating solutions of non-uniform system are very difficult to control and thus are scarcely put to practical use.
Fasteners such as bolts, nuts and washers used for water conduits, gas conduits, plant facilities and the like suffer galling when tightened under severe conditions. Consequently, the tightened connection can not be released, and the fasteners as such have to be cut off. For preventing such troubles, the threads of the fasteners are surface-treated by wet coating (using an oil, etc.), plating, ceramic coating, resin coating or the like in order to prevent galling.
However, when the fasteners are wet-coated, earth and sand are likely to adhere to the coated surface during tightening, impairing the workability. Further, when the wet-coated fasteners are used for water supply conduits, the coating composition may dissolve in and 25 contaminate supplied water. The conventional non-composite plating does not have satisfactory abrasion resistance or durability, and thus easily peels off during the first tightening operation. Therefore, the contemplated galling preventing effect is not achieved. Moreover, both the above wet coating and plating are unsatisfactory in weather resistance and have the drawback that corrosion and other problems occur in the outdoors or in severe environments of high temperature and high humidity. The ceramic coating and resin coating are damaged by the pressure, friction and the like during tightening, failing to exhibit satisfactory galling preventing effect.
In addition, galling is likely to cause also in grinding members such as guide rails for mechanical parts and bearings for rotating parts. In particular, no methods have been found which can effectively prevent galling caused by heavy loads in vacuums which do not permit the use of lubricants, grease, etc., or in semiconductor production facilities wherein prevention of contamination is required.